Transport for NSW said Melbourne to Sydney and Sydney to Melbourne XPT services are both being replaced by a road coaches for the entire journey due to staffing issues.

The regional disruptions come as thousands of commuters across Sydney were told not to travel by train on Monday after negotiations between Transport for NSW, NSW Trainlink and the Rail, Tram and Bus Union (RTBU) broke down.

Union members have walked off the job and Transport NSW said it could not safely operate trains in Sydney.

Transport for NSW Secretary Rob Sharp said while additional buses will be available, the industrial action would leave tens of thousands of people stranded with no opportunity to plan for alternative transport.

"We are doing everything possible to minimise the impact to commuters and sincerely apologise to everyone inconvenienced by this industrial action," Mr Sharp said.

"This was a difficult decision to make but we cannot and will not compromise on safety."

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"We appeared before the Fair Work Commission twice over the weekend, including at 8pm last night [Sunday], seeking a stop to this industrial action by the RBTU."

"These actions will impact hundreds of thousands of commuters and come at a time when people just want to get back to work, back to school and back to university - it's not what our community needs at this time."

The RTBU said the rail chaos was the result of the NSW government's decision to shut the train network to make a point, and not the action of the state's rail workers.

RTBU NSW secretary Alex Claassens said workers turned up to work on Monday morning prepared to take part in low level protected industrial action which would not have impacted commuters, only to be told by management that trains would not be running.

"This is a dummy spit from the NSW government," Mr Claassens said.

"Workers were prepared to take protected industrial action, but only transport management would have noticed the impact, not commuters.

"The impact to services today is not because of workers' actions, but because the NSW government is spitting the dummy and trying to make a point."

The parties are scheduled to again appear before the Fair Work Commission at 9am on Monday to try to lessen the impact of protected industrial action.

Sydney Trains and NSW TrainLink have held more than 30 meetings with the RTBU over the past six months in a bid to reach an agreement on a wide range of claims and remain committed to reach a fair outcome.

Customers are advised to check online at transportnsw.info for the latest updates and if they must travel to utilise alternate modes of transport and allow extra time.

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